Isolation trenches are common features in integrated circuits. Trenches surround devices and isolate one device from another. Bonded wafers, especially bonded wafers made in accordance with U.S. Pat. No. 5,387,555 assigned to the same assignee as this invention, provide a bond oxide layer between a device substrate and a handle substrate. Trenches extend from the surface of the device substrate down to the bond oxide. The trench side walls and floors are then covered with one or more insulating materials. The trenches are filled with a conformal material, typically polysilicon. However, excess insulating material accumulates at the top corners of the trenches and interferes with the filling process. As a result, trenches have voids that extend to the tops of the trenches.
It is common to oxidize exposed surfaces of the device substrate or exposed surfaces of a polysilicon layer in order to convert the device silicon or the polysilicon into silicon dioxide and thereby form a region of isolating material. During a thermal oxidation process, each atom of silicon is joined by two atoms of oxygen. The corner regions at the top of a trench lined with thermal oxide come under stress. Those corner regions increase in volume and the increased volume creates stress in adjacent device area. The stress can result in device defects.
Accordingly, there is an unmet need for an improved process that provides isolation trenches in an integrated circuit without generating stress and device defects. In particular, there is an unmet need for an improved process that reduces device stress and device defects.